Several optical methods have been used over the past decades to investigate thin biomolecular films. Surface plasmon resonance (SPR) spectroscopy is a well-established optical technique commonly used for this purpose.
Surface plasmon resonance (SPR) has gained interest as an efficient mechanism for biosensing. SPR biosensors are sensitive to the refractive index variations occurring within the penetration depth of their evanescent fields. Therefore, an SPR wave is not only sensitive to the variations of the refractive index within a few nanometers of a biomaterial layer on the metal surface (called the adlayer), but also to the variations in the index of the fluid above the adlayer. This cross sensitivity is more pronounced in affinity biosensors where studying the adlayer properties is of particular interest. In general, the aforementioned interfering effects can be attributed to the: adlayer refractive index (na), adlayer thickness (da), and bulk refractive index (nb).
SPR sensors have high sensitivity due to their large field intensity at metal/dielectric interface. Any variations within the penetration depth of the wave can change its propagation constant which results in change in the output signal.
A combination of high sensitivity, label-free detection, and real time sensing make the SPR biosensors the preferred approach for many sensing applications. SPR sensing has been investigated in a variety of sensing configurations such as attenuated total reflection (ATR), grating couplers, optical waveguides, and optical fibers. Among these techniques, ATR using prism coupling is one of the most widely used optical excitation methods in the SPR community.
However, spectroscopy using an SPR wave cannot distinguish between variations in more than one parameter within the penetration depth.
Accordingly, improved apparatuses and methods for performing spectroscopy, or at least alternative, are desirable.